Antenna feed system



Dec. 22, 1959 E. s. LEWIS ET AL 2,918,673

ANTENNA FEED SYSTEM Filed De. 12, 1957 Era 42ml/TH f A A A u @Poramy/@422% im 55d 5 United States Patent @ddee 2,918,673 Patented Dec.22, 1959 ANTENNA FEED SYSTEM Edwin S. Lewis, Pennsauken, and Carl F.Crawford, Delaware Township, NJ., assignors, by mesne assignments, tothe United States of America as represented by the Secretary of the NavyApplication December 12, 1957, Serial No. 702,493

6 Claims. (Cl. 343-786) This invention relates to directional antennaapparatus and more particularly to an improved horn feed for thetransmission and reception of electromagnetic energy.

A common means for producing a pencil shaped beam of transmitted radioenergy comprises a combination of a parabolic reiiector and a pluralhorn feed. The same combination is frequently used to intercept orreceive echoes `from a target. When high gain, during transmission, andhigh sensitivity, during reception, are desired, a reflector having arelatively large diameter with respect to wavelength is required. Foroptimum performance the horn feed is located near the focal point of theparaboloid. The type of plural horn feed generally used has an emissionpattern such that a relatively large ratio of focal length to diameterof the paraboloid is necessary to produce the desired gain andsensitivity. Thus, a large diameter reflector generally has its hornfeed at a considerable distance from its reflecting surface. Thisintroduces a structural problem in that such a horn feed must be rigidlysupported to prevent the feed from becoming misaligned with respect tothe reflector when the antenna is slewed. Beam distortion and faultyreception can result from such misalignment.

If, during reception, a horn feed is used that requires a paraboloidhaving a high ratio of focal length to diameter, the antenna errorpattern will have undesirably high side lobes. In the past, thisundesirable feature has been accepted in order to provide an antennahaving optimum gain. Because of this fact, such antennas, generallyspeaking, will intercept target double echoes from low angle targets,one echo reflected from the target `directly to the antenna, the otherecho from the target being one which is re-reflected into the antennafrom the ground. Since the re-reliected echo has to travel a greaterdistance than does the real echo, it will arrive at the antenna slightlyoutof phase with the real echo. As a result of this phenomenon a falseerror signal can be generated which is sufficiently out of phase withthe proper error signal so as to cause oscillation of the trackingantenna as the target moves.

Accordingly, it is a general object of this invention to provide animproved horn feed which will produce a pencil beam substantially freefrom distortion.

It is another object of this invention to provide an improved pluralhorn feed having an emission pattern such `that it can be disposedcloser to the reflecting surface of a parabolic reflector than hasheretofore been possible.

It is yet another object of this `invention to provide an improvedplural horn feed so designed that the structural problem of supportingthe feed is minimized.

An additional object of this invention is to provide an improved pluralhorn feed so designed as to permit its use `with a parabolic reflectorhaving a focal length to diameter ratio substantially less than asheretofore been required.

It is still another object of this invention to provide a plural hornfeed and parabolic antenna combination of horn portion.

an improved design wherein undesirable side lobes are minimized.

It is a further object of this invention to provide a plural horn feedand parabolic antenna` combination of such improved design that antennaoscillation is minimized when tracking a moving target.

This invention accomplishes these objects by an improved plural hornfeed comprising a waveguide cavity portion feeding into a modifiedbox-type radiating horn portion. The plural horn feed is of generallyrectangular cross section and includes a pair of` normally disposedintersecting dividing walls extending coaxially therein and dividing thewaveguide cavity portion into four substantially equal and parallelwaveguide cavities. The dimensions of the waveguide cavities aresubstantially identical and capable of supporting the propagation oftransverse electric fields of the same mode and orientation in eachcavity. The one of the intersecting dividing walls which isperpendicular to the orientation of the transverser electric fieldspropagated in the waveguide cavities extends through the radiating hornportion of the plural horn feed. The other intersecting dividing wall,which is parallel to the orientation of the iields in the waveguidecavities is terminated at the junction where the waveguide cavityportion feeds into the radiating horn portion. Transmitting energy isfed into and received energy extracted from the wavegude cavities, bymeans of a ywaveguide coupling member to which are coupled a pluralityof rectangular waveguide arms. The coupling between the waveguide armsand the coupling member and between this member and the plural horn feedis entirely conventional and is well known to those skilled in the art.

Radio energy is fed into the waveguide cavities from the coupling memberin a manner such that a transverse electric field of the same mode andorientation is propagated in each of the waveguide cavities. When thisenergy is, in turn, fed into the radiating: horn portion, transverseelectric fields of different modes will be excited therein. For example,when a TEM, mode is paragated in phase in each of the weveguide cavitysections, a TEM, and a TE30 mode will be excited in the radiating Thesemodes will travel down the radiating horn portion at different speedsand, if a proper length of the radiating horn portion is selected, theycan be made to reinforce one another at the center of the aperture ofthe radiating horn portion and will produce a cancelling effect near theedges of the aperture. This reinforcing effect of the modes excited inthe radiating horn portion causes the radiation from it to appear as ifit came from a single conventional horn with a width approximately halfthat of the plural horn feed of this invention. Thus, the radiationpattern from this plural horn feed will be considerably broader thenwould be the case if two or more conventional type horns were employed.The broader radiation pattern in turn, permits the plural horn feed tobe used with a parabolic reiiector having a considerably reduced focallength without any undesirable effect on antenna gain or sensitivity.Another result obtained with an antenna having a shortened focal lengthis a substantial reduction in the ampli-v tude of side lobes and henceimproved target tracking withA minimum antenna oscillation. Finally theshortened focal length also simplifies the problem of properlysupporting the feed at the focal point of the reflector.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as` the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Fig. l is a schematic representation of a plural horn 3 feedillustrating its disposition relative to a parabolic retiector;

Fig. 2 is a fragmentary perspective view illustrating the couplingbetween the waveguide arms and the plural horn feed and having theexterior walls of the coupling member broken away to shown interiordetails;

Fig.. 3 is an end view of a plural horn feed in accordance with thisinvention including a schematic representation of the transverseelectric fields propagated in the feed; l, Fig. 4 is a sectional viewtaken along a line substantially corresponding to line 4-4 in Fig. 3 andschematically illustrating the modes of the transverse electric fieldspropagated in the feed;

`Fig. 5 is a sectional view taken along a line substantiallycorresponding to line 5-5 in Fig. 3 and including a schematicrepresentation of the transverse electric fields 'propagated in thefeed; Fig. 6 graphically represents the emission pattern of the pluralhorn feed of this invention in comparison with the pattern of a feed ofthe prior art; and Fig. 7 schematically presents a comparison betweenthe plural horn feed of this invention (shown in solid outline) with ahorn feed of the prior art (shown in dotted outline). Similar referencecharacters are applied to similar ele- )ments throughout the drawings.In Fig. 1 an antenna reflector 11 is shown having a parabolic reflectingsurface 13 which is to be illuminated v.by the plural horn feed 15 ofthis invention. Supports `17 are provided for the plural horn feed 15.These may /comprise waveguide arms which can also be used to feed energythrough a waveguide coupling member 16 V1:0 the plural horn feed 15. Ifthis is done, the supports 17 will be connected by other waveguides tothe transinitting and receiving apparatus. Fig. 2 shows in greaterdetail the manner in which the waveguide arms 17 support and are coupledto the coupling member 16 and the plural horn feed 15. Two of thewaveguide arms 17 are connected to one end of the coupling member 16while the other two waveguide arms are connected one each to ,the widersides of the coupling member 16. The coupling member 16 feeds into andis attached to the plural horn feed 15.

One method for feeding energy to a plural horn feed is described in thecopending application of John R. Ford and Carl F. Crawford, Serial No.250,726, filed October `10, 1951. Briefly, in the system as explained inthe said Ford and Crawford application, energy is fed to a horn feedantenna array through a single waveguide. Adjacent to and connected to asecond horn of the antenna array is a second waveguide that is coupledby a directiona1 slot coupler to the first guide. To compensate for the90 shift in phase occurring at the slot coupling, a 90 phase shifter isinserted in the second guide between the slot and the antenna feed. Ameans for obtaining the compensating phase shift comprises the insertionof dielectric material in one or both of the waveguides to provide the90 phase shifter. When this is done the dielectric constant andtransverse waveguide dimensions are selected to make the rate of changeof phase shifts with respect to wave length (or frequency) equal at theoperating frequency. Although the above description had reference toonly two waveguides it is equally applicable with four waveguides asused herein.

Briefly, in accordance with the teaching of the above identified Fordand Crawford application, a horizontal dividing wall 29 divides theinterior of the coupling member into upper and lower halves. A verticaldividing wall 28, beginning at a point beyond the coupling of thewaveguide arms 17, further divides the interior of the coupling memberinto four waveguide cavities of substantially equal dimensions. Near thebeginning end of the four waveguide cavities, coupling slots 20 connectthe two upper waveguide cavities with the two lower waveguide cavities.

4 In order to compensate for the phase shift introduced by the couplingslots 20, the two lower waveguide cavities are filled for a portion oftheir length with dielectric material 18. The coupling member 16 isconnected to the plural horn feed 15 and the internal walls 28 and 29are projected into the plural horn feed 15 as will be further describedin connection with Figs. 3, 4 and 5. Alternatively, the coupling member16 and the plural horn feed 15 can be constructed as a unitarystructure.

In Fig. 3, looking into the radiating end of the plural horn feed 15,there are four rectangular waveguide cavities 21, 22, 23, and 24. Eachwaveguide cavity has dimensions capable of supporting propagation of atransverse electric field havingaTEm mode. The dimensions of one cavityare substantially identicalwith those of each of the other cavities. TheTEN modes propagated in the waveguide cavities are represented by thedotted arrows 25.

In Fig. 4 the bottom half .ofthe plural horn feed 15 is illustrated asviewed from a line substantially corresponding to line 4-4 in Fig. 3 andit is depicted as comprising a waveguide cavity portion 26 and aradiating horn portion 27. The top half (Fig. 3) is identicalto thebottom half both in structure and in function. Therefore, thisdescription will be limited to the bottom half as shown in Fig. 4. Twoof the waveguide cavities 23, 24 are shown feeding into the radiatinghorn portion 27. The vertical dividing wall 28 separates these twocavities but does not extend into the radiating horn portion 27.

The TEM, mode propagated in the waveguide cavities 23 and 24 areschematically represented by the curve 30 in cavity 23 and the curve 31in cavity 24 respectively. As the TEN modes 30, 31 travel in phase downthe cavities 23, 24 they are supported by the dimensions of the cavitiesuntil they reach the termination 32 of the vertical dividing wall 28. Atthis point, since there is no wall to support propagation of the twoTEM, modes, they combine to produce new modes. By direct combination aTE30 mode is excited in the radiating horn portion 27 because theinternal width of the radiating horn portion 27 is approximately twicethat of either waveguide cavity 23, 24; a new TEN mode will also beexcited within the radiating horn portion 27. The TEM, mode isillustrated by the curve 33 and the new TEM, mode by the curve 35. Otherhigher odd harmonic modes will also be excited but their effect is sonegligible that they -may be ignored. Over 98% of the energy will existin the TE10 and T1530 modes in the radiating horn portion 27.

At the beginning of the radiating horn portion 27 where the two modesTEN and TE30 (curves 35 and 33 respectively) are excited, they are inphase opposition; The two modes will have different cut off wave lengthsbecause of waveguide effect in the radiating horn portion 27. Because ofthis, they will travel toward the end of the radiating hom portion 27 atdifferent velocities and their relative phases will changeproportionately. By choosing a proper length for the radiating hornportion 27, the two modes can be brought into phase at the terminationof the radiating horn portion 27 as illustrated by the curves 35 and 32.The basic result obtained is to reinforce the energy radiated from thecenter of the radiating horn portion 27 and cancel toward the edges.This results in a radiating horn which appears electrically like onehorn with perhaps one-half the width of the two waveguide cavities 23,24. Because of this electrical effect, a radiation pattern is producedwhich is considerably broader than could be produced by two hornssideby-side each having the same dimensions as the waveguide cavities23, 24.

In Fig. 5, one pair of the upper and lower waveguide cavities isillustrated as viewed from a line substantially corresponding to line5--5 in Fig. 3. Again the plural horn feed 15 is illustrated ascomprising the waveguide cavity portion 26 and the radiating hornportion 27, also the orientation of the TEM) mode propagated in -eachwaveguide cavity is again illustrated by the dotted arrows 25;:` Thehorizontalv dividing wall 29 which separatestthe upper waveguide cavity22 from the lower cavity 23 is' extended into the radiating horn portion27 of the feed. In order to prevent energy from the upper half of theradiating horn portion 27 from being fed back into the lower half andvice versa the horizontal dividing wall 29 is extended a short distancebeyond the termination of the horn portion 27.

As a specific example o-f a plural horn feed constructed in accordancewith this invention, reference is now made to one which was actuallybuilt and tested. A four horn feed was designed for C-band operation inthe frequency range of from 5400 mc. to 5900 mc. A parabolic reflector11 (Fig. 1) having a 12 foot diameter was specifled. The feed had anoverall length of approximately 18finches and the radiating horn portion27 a length of 3% inches. Each cavity in the waveguide cavity portion 26of the feed was .622 inch high and 1.75 inches Wide. The horizontaldividing wall 29 was extended approximately 1/z inch beyond thetermination of the radiating horn portion 27 of the feed. Constructionof a four horn feed having the above dimensions made it possible toemploy a parabolic reflector having a 50.4 inch focal length. In testingthis feed, it was found to have a voltage standing wave ratio of lessthan 1.15, looking into the transmit terminal and produced a pencil beamhaving a half power width of less than 1.2 degrees.

In Fig. 6 the abscissa of the graph represents angles taken on eitherside of the axis bo of the plural horn feed 15 (Fig. 1) and the ordinatedistances represent intensity of radiation from a plural horn feed atthe various angles. The solid curve 41 represents the radiationintensity pattern of a conventional plural horn feed. The dashed curve42 represents the radiation intensity pattern for a plural horn feedconstructed in accordance with this invention. The dashed curve 43represents the intensity pattern of the portion of the energyintercepted by a parabolic reflector and focused into any plural hornfeed to excite an error channel when the antenna system is accuratelytracking an illuminated target. This curve is applicable to plural hornfeeds of the prior art as well as to the plural horn feed of thisinvention.

Comparison of the radiation intensity pattern 41 of a conventional feedwith the pattern 42 of the invented feed shows that the conventionalfeed, in order to minimize side lobes, must have its radiationrestricted to an angle p1 which is much smaller than the permissibleangle p2 that can be used with the invented feed. The smaller radiationangle el is necessary in order to prevent side lobes 44, 45 from beingre-radiated by the reflector. In practice it has been found that withone type of conventional plural horn feed of the prior art the angle p1is limited to approximately 68 while with the improved plural horn feedof this invention a larger radiation angle up to 140 degrees can beobtained.

During reception, a substantially larger portion of the energyintercepted by the reflector is focussed into the feed of this inventionthan is possible with the conventional feed. Because the radiationpattern of the conventional feed is restricted to the angle p1, it has agreater spill-over loss than does the improved feed. This spillover lossis illustrated in Fig. 6 as the area under the curve 43 which fallsoutside the angle p1. Because of the improved reception made possible bythe plural feed of this invention, accurate tracking of low flyingtargets is substantially improved and antenna oscillation during thetracking of moving targets is minimized.

In Fig. 7 the solid outline represents the configuration of a pluralhorn feed 15 and reflector 11 made possible by using the plural hornfeed of this invention. The dotted outline represents the configurationof a plural horn feed 15' and reflector 11 of the prior art. The broaderradiation angle made possible by the improved plural horn feed 15 ofthis invention is again represented by 952 and the smaller angle of theplural horn feed of the prior artas el. In eachinstance-we areconsidering.

that the diameter D of each reflector is the same and of a relativelylarge value to provide for high gain and sensif tivity. Such a diametermight, fo'r example, be 12 feet. In order to properly re-radiate energycorrespondingto the radiation angle 61 of the prior art plural/hornfeed, the feed must be located at or nearthe focal point of a paraboloidhaving a ratio of focal length to diameter (f/D) of about .75. Bybroadening radiation to an angle e2, as is done with the plural hornfeed of this invention, a paraboloid can be used which has an f/Drratioof .35. Because of this fact, it is now possible to sup-A port thefeed 15 by support members 17 which are considerably shorter than the`support members 17 required for the prior art plural horn feed 15. Morerigid support of the plural horn feed 15 is also provided since thesupport members 17 vcan project toward the paraboloid 11 at a far lessacute angle, than is necessary for the support members 17 of the priorart plural horn feed 15".`

Obviously many modifications and variations of the present invention arepossible in the -light of the above teachings. It is therefore to beunderstood that with the scope of the appended claims the invention maybe practiced otherwise than is specifically described.

What is claimed is:

1. In antenna apparatus for transmitting radio energy and including areflector for said energy; a rectangular horn feed for said reflectorcomprising a waveguide cavity portion and a radiating horn portion, atleast one dividing wall in said waveguide cavity portiondisposed'coaxially therein perpendicular to two opposite sides of saidwaveguide cavity portion whereby said waveguide cavity por tion isdivided into at least two separate waveguide cavities of substantiallyidentical dimensions, said dimensions of said waveguide cavities beingdisposed so as to support propagation of transverse electric fieldshaving the same mode and orientation in each of said waveguide cavitieswhereby energy of said mode fed in phase into said radiating hornportion from said waveguide cavities will excite in said radiating hornportion transverse electric fields of at least two modes one of whichcomprises an odd harmonic of the other, said radiating horn portionhaving an electrical lengthtsuch that the portion of the energy of eachmode nearthe axis of said rectangular horn feed will be radiated inphase from said radiating horn.

2. In antenna apparatus for transmitting electromagnetic energy andincluding a reflector for said energy, a rectangular horn feed for saidreflector comprising a waveguide cavity portion and a radiating hornportion of substantially the same dimensions, a pair of normallydisposed intersecting dividing walls extending coaxially within saidrectangular horn feed dividing it into four substantially equal parallelwaveguide cavities, one of said dividing walls extending the full lengthof both said waveguide cavity portion and said radiating horn portion,the other of said dividing walls extending the full length of saidwaveguide cavity portion only, each of said waveguide cavities havingits dimensions disposed so as to support propagation of a transverseelectric field having the same mode in each of said waveguide cavitiesand oriented within each said waveguide parallel to said one of saiddividing walls whereby energy of said mode and orientation fed into saidradiating horn portion from said waveguide cavity portion will excite insaid radiating horn portion transverse electric fields of at least twomodes, one of said two modes comprising an odd harmonic of the other,said radiating horn portion having an electrical length such that theportion of the energy of each of said two modes near said one of saiddividing walls will be radiated in phase from said radiating hornportion.

3. In `antenna apparatus for transmitting electromagnetic energyincluding a parabolic reflector,I a rectangular horn feed disposedsubstantially at the focal point of said reflector; said plural hornfeed comprising a waveguide cavity portion and a radiating horn portion,at least one dividing wall in said waveguide cavity portion disposedcoaxially therein perpendicular to two of the sides of said rectangularhorn feed whereby said waveguide cavity portion is divided into at leasttwo separate waveguide cavities of substantially identical dimensions,said dimensions being such that each waveguide cavity is capable ofsupporting a transverse electric field having a TEM, mode, saiddimensions being disposed such that the TEN mode n one of said waveguidecavities is oriented parallel with the TEM, mode in every otherwaveguide cavity, whereby when at least two TEM, modes are fed into saidradiating horn portion from said waveguide cavities at least one newTEN, and one TES mode are excited in said radiating horn portion, saidradiating horn portion having an electrical length such that said newTEM, mode and said TE30 mode will radiate from said radiating hornportion in phase along the axis of said rectangular horn feed.

4. In antenna apparatus for transmitting electromagnetic energy thecombination of a parabolic retiector having a focal length to diameterratio of substantially .35 and plural horn feed of rectangular crosssection for said refiector disposed substantially at the focal pointthereof, said plural horn feed comprising a waveguide cavity portion anda radiating horn portion, at least one dividing wall disposed coaxiallywithin and perpendicular to two opposite sides of said waveguide cavityportion whereby said waveguide cavity portion is divided into at leasttwo waveguide cavities of substantially identical dimensions, saiddimensions having Values and disposition such that each said waveguidecavity will support the propagation of a transverse electric fieldhaving a TEN mode and that the TEm Inode propagated in one of saidwaveguide cavities has the same orientation as the TEN, mode in eachother waveguide cavity, whereby energy of said T1510 mode fed in phaseinto said radiating horn portion from each of said waveguide cavitieswill excite in said radiating horn portion at least two transverseelectric fields one of said fields having a TEw mode and another havinga TESO mode, and said radiating horn portion having an electrical lengthsuch that said TEN and TE30 modes are radiated in phase from said pluralhorn feed along the axis thereof.

5. In antenna apparatus for transmitting electromagnetic energy thecombination of a parabolic reector having a focal length to diameterratio of substantially .35 and a rectangular horn feed for saidreflector disposed substantially at the focal point thereof, said feedcomprising a waveguide cavity portion and a radiating horn portion ofsubstantially identical dimensions, a pair of normally disposedintersecting dividing walls extending coaxially within said rectangularhorn feed dividing it into four parallel waveguide cavities ofsubstantially equal cross section, one of said dividing walls extendingthe full length of said waveguide cavity portion only, the other of saiddividing walls extending the full length of both said waveguide cavityportion and said radiating horn portion, each of said waveguide cavitieshaving dimensions capable of supporting propagation of a transverseelectric field having a TEM, mode, said dimensions being disposed suchthat each said TBI@ mode is oriented parallel to said one dividing wall,whereby feeding energy of said TEM, mode in phase from each of saidwaveguide cavities into said radiating horn portion will result in theexcitation of at least two transverse electric fields in said radiatinghorn portion, one of said fields having a new TEM, mode and another ofsaid fields having a T E30 mode, said radiating horn portion having anelectrical length such that said new TEN mode and said TEBO mode areradiated in phase from said rectangular horn feed along the axisthereof.

6. In antenna apparatus for transmitting electromagnetic energy andincluding a reflector for said energy, a plural horn feed for radiatingenergy onto said reector comprising a plurality of means for supportingthe propagation of transverse electric fields of one mode polarized in asingle direction, means for combining said fields to produce atransverse electric field of said one mode and at least one othertransverse electric field of an odd harmonic of said one mode and meansfor directing said fields onto said reflector from a point where thecenters of said one mode and said odd harmonic are substantially inphase.

No references cited.

